The present invention relates to the production of a printing block or form of the kind in which at least one original is scanned optoelectronically to obtain image signals these being mixed to generate a composite signal which is recorded as the printing block or form. One of the image signals which is to be mixed is modulated by a control signal and the other image signal is modulated by the amplitude-inverted control signal the modulated image signals being summated to obtain said desired composite signal, said control signal determining a mixing range and the mixing characteristic within said mixing range. Hereinafter such a method of producing printing blocks or formes will be referred to as "of the kind described".
Such a method was introduced to satisfy the demand for gradually extending transitions between one picture and another which frequently arises in copying pictures in mutually interposed positions. It is equally desirable in the case of a variety of corrective tasks, to allow the action of a correcting operation to set in or die out gradually.
The evolution of the control signal in a method of the kind described determines the extent of the mixing range and the transitional characteristic of the signals which are to be mixed. The signal in question is a two-level signal per se, which gradually rises or sinks within the mixing range.
A control signal of this nature, hereinafter referred to as a diffuse control signal, is obtained in a prior proposal by low-definition scanning of a control mask by means of an ambient field diaphragm, the change of the signal level along a mask outline not occurring suddenly but gradually or smoothly. This evolution of the control signal may also be derived by sharply focussed scanning of the control mask under simultaneous evaluation of the picture dots surrounding the picture spot actually scanned (ambient field) or by evaluating particular properties (colouration, tonal value, etc.) of the image original.
In an embodiment according to the said prior proposal, a black/white control mask is scanned. The application of a black/white control mask has the disadvantage however that one control signal only may be obtained unless use is made of several such control masks and several scannings elements, which would be very costly. The reader's attention is directed to co-pending application Ser. No. 879,596.
The problem inherent in controlling a number of smooth transitions at the same time, e.g. if it is intended to copy several image originals in mutually interposed positions at the same time and/or to perform several colour or tonal value corrections in different areas of the originals at the same time.
It is already known from U.S. Pat. No. 3,621,126 that several control signals may be obtained by scanning a coloured control mask, for copying images in mutually interposed positions. This known system however provides sharply defined control signals only, i.e. signals incorporating a sudden variation of the signal level, so that it is inappropriate for obtaining the required evolutions.
The manner in which a definition signal may be switched adjunctively and disjunctively for particular areas of the image original as a function of specific properties (contrast, brightness, colour) of the image original or of a control mask, is furthermore described in U.S. Pat. No. 4,005,475. The action of the definition signal on the image signal may however be controlled in irregular manner only and not whilst setting in or diminishing gradually, even with this known circuitry.
To this must be added that sharply defined as well as gradual transitions are frequently required. For example, pictures and textual matter are to be copied in mutually interposed positions in sharply delimited manner and different gradual corrections are to be performed at the same time.